Hamsters infected in utero through injections of their mothers with the parvovirus H-1, develop craniofacial anomalies with predictable regularity. We propose, by means of immunofluorescent techniques as well as histological and electron microscopic studies, to trace this teratogenic virus from the time and site of maternal injection to the embryonic cells affected. We hope in this manner to determine crucial areas in the developing embryo involved in the induction of anomalies before any change in gross morphology can be observed; we may also be able to pinpoint regions of importance where anatomical aberrations per se are not seen. A large series of pregnant hamsters will be injected on each day of the gestation period of 16 days so that the effect of the virus infection on the developing embryo from the time of coitus to delivery can be assessed - in particular as such effects relate to the induction of craniofacial deformities.